This invention relates, in general, to semiconductor packages, and more particularly, to ball-grid array semiconductor packaging.
Ball-grid array (BGA) semiconductor packages are well known in the electronics industry. BGA packages provide denser surface mount interconnects than quad flat pack (QFP) packages. Industry consensus is that BGA packages are more cost effective than QFP packages for input/output (I/O) requirements greater than 250. However, there is a great demand for cost effective BGA solutions down to 100 I/O.
During the assembly of a BGA package, an organic resin printed wiring board substrate having a thickness on the order of 0.35 millimeters (mm) is placed on a metal pallet or support device. The metal pallet provides support for the printed wiring board during the majority of assembly steps. The printed wiring board comprises a single BGA substrate or a single row or strand of a number of BGA substrates. The largest available single strand printed wiring board is a 1xc3x976 printed wiring board with a maximum total length of about 200 mm. Next, a semiconductor die having a multitude of bonding pads is attached to a die pad located on the top side of the BGA substrate. Wire bonds are then attached to the bonding pads and to bond posts on the top side of the BGA substrate. Next, the semiconductor die and the wire bonds are encapsulated with an organic material. After encapsulation, the encapsulation material is cured at an elevated temperature. Conductive solder balls are then attached to contact pads, which are on the lower side of the BGA substrate and electrically coupled through conductive traces to the bond posts, using a solder reflow process. Each BGA package is then marked. When a single strand of multiple BGA packages is used, a singulation process such as a punch press is used to separate the multiple BGA packages into single units.
The above assembly process has several disadvantages. Because the above process requires a metal pallet to support the thin BGA substrates during the majority of assembly steps, the process is not conducive to large scale automated assembly. As a result, manufacturers must purchase additional equipment to assemble BGA packages. This requires capital investment in equipment and additional factory space. Also, because only single substrates or a single strand of a several substrates is used, it is difficult for manufacturers to produce a large volume of BGA packages efficiently. In addition, the above process requires significant labor inputs to load and unload the metal pallets or support devices at the various process steps. This negatively impacts manufacturing cycle time and quality. Furthermore, the pallets are expensive because they require precise tolerances for use with automated equipment and they require a manufacturer to carry a large inventory to support work-in-process (WIP) throughout a manufacturing line.
Industry standards require that after assembly, each BGA substrate must maintain a planarity variation of less than approximately 0.15 mm (approximately 6 mils) as measured at three points across a substrate. In other words, each BGA substrate must not be excessively warped or non-planar. Because of this strict standard and a concern over warpage, printed wiring board suppliers and BGA semiconductor manufacturers have not been motivated to expand beyond the existing 1xc3x976 single strand printed wiring board.
With the rapid increase in demand for BGA packages, it is readily apparent that a need exists for cost effective printed wiring board substrates that are conducive to large scale automated assembly, that support existing automated assembly equipment, and that do not warp during the assembly process.